RS

Reset-dominant bistable function block. A flip-flop where the reset input takes priority: if both S and R1 are TRUE, the output Q1 is FALSE.

IEC 61131-3	Section 2.5.2.3.1
Support	Supported

Inputs

Name	Type	Description
S	BOOL	Set input. Sets Q1 to TRUE while TRUE, unless R1 is also TRUE.
R1	BOOL	Reset input. Clears Q1 to FALSE while TRUE; takes priority over S.

Outputs

Name	Type	Description
Q1	BOOL	Latched output state. Retains its value between scans.

Behavior

The output is computed as Q1 := NOT R1 AND (S OR Q1). When R1 is TRUE, the output is cleared regardless of S. When only S is TRUE, the output is set. The output retains its value between scans (latching).

Example

This example shows that reset dominates: both S and R1 are TRUE, yet output is FALSE.

Try it

Source

PROGRAM main
   VAR
      latch : RS;
      output : BOOL;
   END_VAR

   latch(S := TRUE, R1 := TRUE, Q1 => output);
   (* output is FALSE because reset dominates *)
END_PROGRAM

See Also

• SR — set-dominant bistable